An optical navigation device, such as an optical mouse, conventionally uses a light-emitting diode (LED) to broadcast light onto a surface. The light is reflected off the surface to a sensor. The reflection of the light produces a pattern that is captured by the sensor. The sensor can include, for example, a complementary metal-oxide semiconductor (CMOS) imager array. As an optical navigation device is moved relative to the surface (or vice versa), successive images (frames) are rapidly captured and compared to measure the amount of movement. The difference between successive images indicates the amount of movement. Optical navigation devices such as those described herein operate at frame rates of 1500 frames per second or more. Thus, relatively small amounts of movement can be detected and measured.
The amount of time needed by the optical navigation device (specifically, the sensor/imager) to capture an image is referred to as the exposure time. For example, on a difficult surface such as a dark surface, a longer exposure time may be needed to capture images at a resolution that is high enough to discern movement of the optical navigation device. Conventional devices use the “squal” (surface quality) value to determine exposure time. A squal value can be calculated for each frame. When the squal value decreases, the exposure time is increased.
A problem with conventional optical navigation devices is that the LED light source can degrade over time. The amount of light emitted by the LED, referred to herein as the brightness of the LED, can decrease with time. The reduction in brightness can cause a reduction in the squal value and hence an increase in the exposure time.
As exposure time is increased, frames are captured less frequently, which has a deleterious effect on the tracking performance of the optical navigation device. With longer exposure times, some movements may not be detected or the movement of a cursor may appear less smooth, for example.
The impact of a longer exposure time on tracking performance may be noticeable on a good surface, but it is especially noticeable on a difficult surface. That is, the exposure time may already be increased to cope with a difficult surface, and the decrease in LED performance can lead to a further increase in exposure time and a greater impact on tracking performance.
In summary, the performance of an optical navigation device can be diminished by degradation of the LED. Small movements of the device may not be detected. Movement of the cursor on a display screen may be more jumpy in appearance. As the LED degrades, the user can become less satisfied with the performance of the optical navigation device and may be forced into the purchase of a new device. Therefore, an optical navigation device that addresses the problems caused by LED degradation would be advantageous.